Stanene the new gas sensing wonder material: current status and future prospects

In the search for new 2D materials beyond graphene with similar exemplary properties Stanene the new graphene relative was successfully synthesized and characterized in 2016 [14]. This new material which consists of atomically thin tin (Sn) atoms arranged in a hexagonal lattice has shown great promise in gas sensing applications in its pristine, doped and functionalized forms as evidenced by the recent research outputs following its discovery. Researchers have discovered that with its exotic properties it is highly efficient in the detection of the environmentally harmful gases like SO2, NO2, CO2, NH3 and can serve as a multifunctional gas sensor. In this focused and up-to-date review we aim to group the literature on first principles studies of stanene regards to its application for gas sensing and critically examine the success/failures of its multifunctional gas sensing properties as compared to graphene. We systematically present and discusses how structural deformations, defects and doping effect the gas sensing performance. Despite the impressive progress in the field of gas sensing with stanene shown by first principles calculations, many challenges in large scale synthesis, device fabrication and commercialization need to be addressed. The current review can help understand the current status and drawbacks of stanene and identify future directions in this field.


Introduction
Gas adsorption and sensing is very important in today's increasingly industrialised world with the presence of so many harmful gases like SO2, NO2, N2O and NH3 detrimental to the environment and human well being. Accurate detection of these toxic gases as well as the monitoring of CO2, O3 and O2 levels in the atmosphere require selective and sensitive sensors; and nanomaterials with their high surface to volume ratio are very attractive offering large surface areas for adsorption. The last two decades has seen a lot activity in finding suitable nanostructured materials for gas sensors and graphene (Gr) and Gr like 2D materials have played a major role in this aspect [1][2][3][4][5]. The exemplary performance of Gr and its exotic properties and the immense possibilities available with the different nanostructures single, bi and few layers of Gr, CNTs, nano-ribbons and nano-composites for diverse applications in optoelectronics, energy, desalination, sensing [6][7][8][9][10] and other fields has led to a race to find new Gr analogues. The quest for other elemental analogues of Gr has resulted in various studies on Antimonene, Arsenene, Bismuthene, Borophene, Germanene, Indiene, Phosphorene, Silicene and Stanene [11][12][13]. Among these Stanene a 2D hexagonal lattice of Sn (Tin) atoms was successfully synthesized and investigated by Saxena et al in 2016 [14]. They were able to synthesize from mono to few atomic layers of free standing stanene and characterize them optically using UV-Vis absorption, Raman and photoluminescence spectroscopy and prove experimentally its existence and stability. Their VASP DFT first principles calculations suggest that stanene stabilizes in low buckled configuration with buckling height of ∆=0.85Å, in good agreement with previously reported data [11]. The buckling causes an overlap of the π and σ orbitals and weakens the π-π bonds. Table I lists the physical and electronic properties of Stanene in comparison with Gr.  [11], [14] Single planar atomic layer of C atoms in hexagonal honeycomb lattice [15] Bonding Sp 2 and sp 3 ; weak π-π bonding [11] strong covalent sp 2 bonds; [16] Band gap 73 meV with SO; Topological insulator [17] Gapless semiconductor [16] Spin orbit coupling Strong weak Dielectric constant ε⊥ and ε∥ are ∼1.6 and ∼14.4 respectively [18] ε⊥ and ε∥ are ∼3 and ∼1.8 respectively @low E fields [19] Mobility 1297.24 and 1591.53 cm /Vs in the x and y directions.
[20] 15,000-200,000 cm /Vs [16] Conductivity 100% @room temperature [21] < 100% [22] Thermal conductivity 11.6 W/mK [23] Up to 4000 W/mK [24] Mechanical strength 25.2 GPa [25] 130GPa [26] As seen from the Table, the room temperature topological insulator stanene is a novel 2D material that can overcome the deficiencies of Gr. The excellent electronic properties makes stanene stand out with its exceptional gas sensing performance as shown by the recent investigations of this material by various groups in its pristine, structurally defected and doped forms [27][28][29][30][31][32]. Stanene appears to be the new 'gas sensing wonder' and it is time to review and recapture the major achievements and progress made in view of the highly intensive theoretical investigations in this topic during the past five years. This review therefore aims to discuss the latest advancements from the literature survey of the recent first principles studies of stanene regards to its application for gas sensing. We first elaborate the synthesis methods followed by gas-sensing mechanisms used and introduce the various types of gas-sensing devices. We then describe the structural parameters and other factors like defects and doping that influence the gas sensing performance. The current status and drawbacks of stanene for sensing of SO2, CO, CO2, NO2, NH3 and other harmful gases is examined and compared with graphene. Finally the future prospects and challenges of this cousin of Gr for gas-sensing applications is presented. The review is arranged according to the main sections: 1. Introduction; 2. Methods for synthesis; 3.Sensing mechanisms; 4.Gas sensing functionalization; 5. Performance; and 6. Challenges and Future outlook.

Synthesis methods
The fascinating predicted properties of stanene as a 2D topological insulator and its excellent gas sensing capabilities is a stimulus for the synthesis of stanene and its derivatives and further experimental investigations. Group-IV monolayers (ML) have been experimentally realized using various experimental techniques and theoretically characterized in the past few years. [33][34][35][36]. Compelling evidence of graphene like 2D silicon was provided by Vogt et al [33] with the synthesis of epitaxial silicene sheets grown on Ag(111) substrate and by Fleurence et al [34] on diboron films. First principles method for characterization of new materials is a powerful and accurate tool for prediction of properties. It can also be used for modeling ideal growth mechanisms and substrates. Guo et al explored in 2016, the potential of Ag (111) surface as an ideal substrate for the epitaxial growth of monolayer (ML) stanene [37]. Using first-principles calculations, they studied the stability of the structure of stanene and the diffusion behavior of Sn adatom on Ag(111) surface in different epitaxial relations with respect to Ag(111) surface. Their major findings that the hexagonal structure of stanene ML is well preserved on the Ag(111) surface and that the height of epitaxial stanene ML is comparable to the step height of the substrate supports the fact that it is possible to achieve large-area growth of stanene. Figure 1 reproduced from their work indicates that the diffusion barrier of Sn atom on Ag(111) surface is not only low but also uniform, providing excellent conditions for the large-scale epitaxial growth of stanene ML on Ag(111) surface even at low temperature. Once, the epitaxial stanene ML is detached from Ag (111) surface perfect lattice structure of free-standing stanene can be achieved. In addition, their simulated STM images with the height line profiles can provide important references for future experiments. In the recent years 2D buckled films were grown on metals similar to Ag such as germanene on Au [38] and Pt [39], borophene on Cu [40] and silicene on Ir [41]. These metals could also be good match as substrates for the epitaxial growth of stanene; further verification and experimentation are required.
Recently in 2015, ML and few-layers (FLs) stanene were successfully grown on Bi2Te3 (111) surface using molecular beam epitaxy (MBE) and the atomic structures and their electronic properties were studied using scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations [42]. It was found that the stanene epitaxial growth on Bi2Te3(111) was a mixture of ML, bilayer (BL) and FLs. The reason could be the sharp steps of Bi2Te3(111) surface ~1 nm in height, which are able to block the continuous growth of stanene by suppressing the growth fronts steps. This proves the critical role the substrate plays in the growth mechanism and proper choice of substrate is essential for good epitaxy of stanene ML.

Sensing Mechanisms
The honeycomb structure of the two dimensional Gr and Gr analogues gives them exceptional electronic, mechanical, chemical, and physical properties mainly coming from the quantum size effects [11,[43][44][45] as summarized earlier in Table1 for Gr and stanene. A change in the chemical environment by the introduction of a new chemical species or functionalization introduces a change in these properties and can form the basis of detection and sensing mechanisms. In Gr and layered 2D inorganic Gr analogues the main sensing mechanism is based on the charge transfer processes, in which the 2D sensor material acts as charge acceptors or donors. When exposed to different gases, there is a charge transfer process that occurs between the sensor and the adsorbed gases, with different magnitudes of charges and direction dependent on the nature of the gas species and 2D sensor interaction.

Charge transfer
The presence of gas/gases changes the chemical environment of the sensor-analyte system results in electronic charge density difference (CDD) due to the charge transfer. A measure of the CDD gives a direct qualitative measure of the gas adsorption. Figure 2 shows an example of a charge transfer mechanism, taken from the work of Yue et al [46] on gas sensing using MoS2 ML, a Gr 2D analogue. The Gases O2, H2O, NO, NO2 and CO all act as charge acceptors as seen from the figure. Only NH3 acts as a charge donor. This same behavior of the gases is what was observed for Gr and its derivatives and it is also expected with other Gr analogues sensing materials due the similarities in their structures. In fact this is confirmed in the works of Tang et al [47] for Co anchored Gr and Sv-Gr (Gr with single sulphur vacancy), Abbasi & Sardroodi [28] for pristine and boron doped stanene and the boron doped stanene work of Garg et al [27]. The charge transfer that takes place during the interaction of the senor material and gas will also change the resistance of the system, and resistance based measurements can also be used to estimate the gas adsorption which is discussed in the next section.

Resistance based
Indirect way of estimating the gas adsorption by measuring the change in resistance, or by noting the change in the optical/photoluminescence spectra of the system, which are a result of the electronic charge/spin density change upon the interaction of the 2D sensor with the gas.
Resistance based sensor devices could be chemiresistors [48,49], field effect transistors FET [50,51], Schottky diode [52], Impedance sensors [53,54], conductometric semiconducting gas sensors [55][56][57], Surface acoustic wave (SAW) devices, Surface work function (SWF) [58][59][60], or heterojunction sensors [61]. The choice of the sensor device is dictated by the user requirements on selectivity, sensitivity, cost, the analyte and other considerations. Chemiresistors are easy to fabricate, simple to use, with low power consumption and ability for reuse; in addition they are cost effective. In comparison FET sensor devices are portable, simple to fabricate, sensitive with nanoscale miniaturization. They can be used to sense many kinds of gases, such as CO, NO, NH3, NO2, SO2, H2 etc [62][63][64]. Figure 3 shows the schematics and transport characteristics of a FET gas sensor device used for sensing NO2 and NH3 with rGrO as the conducting channel between the source and drain electrodes, and with Si as a back-gate electrode reproduced from the work of Lu et al [65]. The sensor has a response of 100 ppm to NO2 in air. Impedance sensors are usually composed of a noble metal, a metal oxide, and layered materials and utilize solid-state impedance spectroscopy to measure the sensing response over a wide range of frequencies from sub-hertz to megahertz. It can accurately detect gases at the one part per million. Low concentrations of various gases including hydrocarbons, NOx, CO, and humidity can be detected using these types of gas sensor devices. An impedance gas sensor that selectively detects methanol based on layered black phosphorus and a gold electrode is presented in Figure 4 which is adapted from the work of Mayorga-Martinez et al [66].

Gas sensing functionalization
The sensing performance of 2D materials can be improved by functionalization; these could be physical/structural or chemical. A change in the structure of the material by the application of strain, pressure, temperature or nanostructuring (ML,BL,NTs) will cause a change in the electronic properties of the material that can modify the adsorption characteristics. Another way of modifying the electronic properties is through chemical functionalization i.e. decorating/doping with elements of another species or the introduction of defects/vacancies. All these factors effectively change the electronic charge distribution, density of states (DOS) and band structure of the 2D material and tune the properties to our liking. Figure 5 gives a snapshot of the functionalities that will be discussed and reviewed in relation to the gas sensing capabilities of Gr and stanene.

Structural deformations (vacancies, defects, strain)
Strain is an important parameter that will change the structure of a material and causes changes in the bandgap and electronic properties. Modarresi et al. [67] used DFT and molecular mechanics models to study the effect of external strain on stanene with spin orbit coupling (SOC) taken into account. The SOC is important for Sn as it is an heavy metal, and it opens the zero-gap of stanene to a gap of 70 meV and makes it a topological insulator. One of the key issues in high performance FETs operating at room temperature is the presence of a sizeable gap. The sensing performance is therefore enhanced. Upon applying external strain Modarresi and his co-workers observed a closing of this gap and the electronic structure changes that are not favourable for gas sensing. They also studied the size and chirality effects of stanene nanoribbons. Kumar et al [68] have studied the effect of the role of external defects in the gas sensing capabilities of Gr chemical FETs. They show that the sensitivity (in terms of modulation in electrical conductivity) of pristine Gr is actually due to external defects in the insulating substrate which can modulate the electronic properties of Gr. This is shown in Figure 6 reproduced from their graphical abstract.

Non-metal doping
The effect of non-metals boron and N doped stanene on gas sensing of NO, NO2, NH3 and N2O have been studied by Garg et al. [27] using DFT and Dispersion corrected (DFT-D3) functionals and spin orbit coupling. Using Bader charge analysis and CDD they show that doping improves the interaction between stanene and gas molecules. They observed that the doped systems (B@-, N@-, and B-N@stanene) show better sensitivity and selectivity than pristine stanene. Abassi et al [28] also investigated boron doping and examined the adsorption behaviors of triatomic gas molecules (NO2, SO2 and CO2) on the buckled stanene nanosheets. Their Mulliken charge and CDD calculations indicated that the adsorption of gas molecules on the doped stanene nanosheets is energetically more favourable than that on the pristine ones in agreement with Garg et al [27]. Although group IV-2D structures silicene, germanene, and stanene have been realized experimentally and have rather high carrier mobilities they are all semimetallic without an appreciable band gap, which makes them unsuitable for FET applications. In an attempt to integrate them in microelectronics recently, Bianco et al [69] successfully fabricated fully hydrogenated germanene (germanane) via a chemical method. Interestingly, germanane is semiconducting with a moderate band gap as well as a considerable carrier mobility greater than 10 4 cm 2 V -1 s -1 . Hydrogenation seems effective in opening the gap and increasing the gas sensing capabilities. Encouraged by the work of Bianco et al, hybrid density functional theory (DFT) computations were carried out to investigate the structural, electronic, and optical properties of hydrogenated stanene, namely stanene (SnH) by Liu et al [70]. Their computations revealed that stanane is semiconducting with a direct band gap of 1.00 eV and can be flexibly tuned by applying an external strain. Moreover, they found that stanane has rather strong optical absorption in the visible as well as infrared regions of the solar spectrum and electron/hole nobilities higher than that of MoS2.

Metal doping
Stanene has been doped with several metals and transition metals in various first principle studies; Zhang et al investigated the incorporation of transition metals Ni, Cu and Au in stanene ML for gas sensing. They found that the Ni-adsorbed stanene monolayers were semiconductor in nature, while the Cu and Au-adsorbed ones exhibited metallic property. They examined various adsorption sites and directions for the binding of gas molecules to the Au-adsorbed stanene systems and found that the Au adsorption enhances the adsorption of gas molecules on the stanene system. Their CDD analysis indicated that the bond in the Au-embedded stanene with adsorbed gas molecules is of a covalent nature due to the location of the favoured electron accumulation sites which are largely at the middle of the Au-gas molecule bond. They predict Au-adsorbed stanene systems as superior candidates for gas sensing. The interaction of some gas molecules (CO, NO, N2O and NH3) with Pd-decorated stanene nanosheets using DFT was studied recently by Abassi [71]. Three different patterns for embedding Pd into the stanene ML was considered in the work before placement of the gas molecules on the Pd-decorated systems. It was shown that based on Mulliken charge analysis, the positive charge transfer occurred from the gas molecules to the Pd-decorated stanene systems and that pattern-III Pd-embedded stanene ML is the most energetically favourable one as shown in Figure 7 reproduced from this recent work. Figure 6.Isosurface plots of the electron charge density difference for pattern-III Pd-decorated stanene monolayers with adsorbed CO and NH3 molecules (isovalue is ± 0.0002 a.u). [71]

Performance
Sensitivity, selectivity, stability, response, and recovery time are the main parameters that govern the performance of a sensor. The sensor's response can be defined as the % change in sensing property (with and without the presence of gas molecules) with respect to the original sensing property value of the sensor alone. As one of the important parameters of gas sensors, sensitivity (S) can be defined as the measured signal change induced by per concentration unit of gases, and this value can be extracted from the slope of the curve based on the electrically measured parameter (resistance, voltage) vs the gas concentration in resistance based sensors. Selectivity on the other hand is the capability of a gas sensor to respond to a specific group of gases or even to a certain gas in the presence of other gases. Usually, sensors are sensitive to more than one gas at the same time. Selectivity can be measured or estimated by comparing the effects of different gases on a sensor. Selectivity of the gas sensor is necessary for many applications and cross-sensitivity should be avoided. The stability of a gas sensor characterizes the quality of a signal over a period of time and indicates the reliability of the senor measurements. The response (time to reach 90% of response in the presence of gases) and recovery times (the time to return to 90% values in the absence of gases) are important key parameters of performance of a gas that are measured in the range of seconds to several tens of minutes. So far, since all the gas sensing properties of pristine and doped stanene has been predicted from first principles using charge transfer, CDD, band structure analysis there are only qualitative results showing good, better, best or bad performance sensor ratings.

Challenges and future outlook
In conclusion, stanene the new young Gr cousin has great potential as a gas sensing device material in its pristine and doped forms as evidenced from the recent first principle calculations. This room temperature topological insulator is a novel 2D material that can overcome the deficiencies of Gr. with its excellent electronic properties. Stanene stands out as the new 'gas sensing wonder' with its exceptional gas sensing performance. However there are quite a few challenges to be overcome. Although, it has been experimentally synthesized much work needs to done to reach the state of the art sophistication in large scale synthesis of stanene comparable to Gr and 2D MoS2. Device fabrication and testing are required to experimentally verify the theoretically predicted results. More intense research theoretically and practically is needed to form guidelines for stanene based sensors device fabrication that can make quantitative measurements and establish its superior gas sensing performance without doubt. Temperature and humidity also play important roles in the sensing performance of 2D materials and need to be investigated thoroughly. Repeatable, reliable, highly sensitive, selective and cost effective are the criteria that the future stanene based sensors need to address.